1. Field of the Invention
The present invention pertains generally to printing units of a printing press and more particularly to an improved system for mounting the cylinders of such printing units.
2. Description of the Prior Art
Conventional web offset perfecting printing units include two printing couples, each couple comprising a plate cylinder and a blanket cylinder. The plate cylinder of each couple is disposed in rolling engagement with the blanket cylinder whereby it transfers an ink impression onto a blanket cylinder. The blanket cylinder, in turn, transfers the ink to the web material with which it is in rolling contact in the form of a printed image. The second printing couple likewise includes a second plate cylinder and a second blanket cylinder which prints on a second side of the web of material simultaneously with the printing on the first side by the first blanket cylinder.
As is well known, it is necessary for purposes of accessibility as in changing plates and during web breakage and paper wrapup, to separate or "throw off" the printing cylinders. To that end, it has been common practice as disclosed, for example, in U.S. Pat. No. 2,874,636, to mount at least one cylinder of each printing couple, and generally three of the four cylinders in throw offs comprising a complicated system of eccentrics, bearings, levers and links. In order to maintain the degree of blanket pressure required to produce a quality printing job, it has been necessary to precisely maintain the positions of the cylinders. This was accomplished in prior art devices by using very stiff mounting mechanisms, requiring the eccentric boxes and cylinder bearing housings to be precisely machined and thus very expensive.
As will be hereinafter explained, such stiff mounting mechanisms are subjected to severe stresses during the printing operation with resultant high maintenance costs and excessive down time. Maintenance of precise cylinder positioning has been greatly facilitated by the addition of cylinder bearer supports as taught, for example, in U.S. Pat. No. 3,256,812. Such bearer supports or rings are fitted to the shafts of the cylinders and are of such diameter relative to that of the associated cylinders that the bearer rings of one cylinder rotate in engagement with the bearer rings of an adjacent cylinder when the cylinders are in the "thrown on" or printing position. The bearer rings serve, among other things, to regulate the spacing between the cylinders without creation of excessive pressure between the cylinder surfaces themselves. By interconnecting the adjacent cylinders, the bearer rings have significantly improved the stability of color register, assisted in defining proper blanket squeeze and, particularly in single-width presses with hollow cylinders, markedly reduced cylinder bouncing which is the major cause of streaking.
While the addition of bearer rings in the aforedescribed manner represented a significant advance in this phase of the printing art, the improved device did not entirely avoid the problems encountered in support of the cylinders in that the stiff bearing supports, wherein pre-loading is applied through the cylinder bearing housings, were still employed. The use of stiff or rigid bearing supports in combination with stiff bearer links or rings tends to be self-destructive inasmuch as the arrangement is very sensitive to bearing and/or bearer eccentricities or runouts. As the stiffness of the supports is increased, the potential for greater sprious forces of self-destruction are introduced.
It is believed the major cause of large runouts is severe web wrapup following web breakage. The wrapup results in relatively large residual deformation of bearing and bearer ring surfaces as well as bending of the cylinder journals. With the degree of stiffness or rigidity required in the bearing supports of such a system to achieve the desired bearer precompression and blanket squeeze, severe web wrapups results in irrevocable loosening of bearings leading, in turn, to bearer separation. The bearers thus become idle or ineffective since their function is to operate in rolling engagement with one another and thereby establish and maintain proper cylinder position and blanket pressure.